[sksg] Refactor inval registration

... to avoid having too many Node friends.

TBR=
Change-Id: I8f8ff570d94ea48017935066a3d51cd8265ec120
Reviewed-on: https://skia-review.googlesource.com/97980
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/experimental/sksg/effects/SkSGMaskEffect.cpp b/experimental/sksg/effects/SkSGMaskEffect.cpp
index d4ce6df..17b4da7 100644
--- a/experimental/sksg/effects/SkSGMaskEffect.cpp
+++ b/experimental/sksg/effects/SkSGMaskEffect.cpp
@@ -14,11 +14,11 @@
 MaskEffect::MaskEffect(sk_sp<RenderNode> child, sk_sp<RenderNode> mask)
     : INHERITED(std::move(child))
     , fMaskNode(std::move(mask)) {
-    fMaskNode->addInvalReceiver(this);
+    this->observeInval(fMaskNode);
 }
 
 MaskEffect::~MaskEffect() {
-    fMaskNode->removeInvalReceiver(this);
+    this->unobserveInval(fMaskNode);
 }
 
 void MaskEffect::onRender(SkCanvas* canvas) const {